A typical wireless lighting array comprises a large number of electrically driven luminaires, which are typically arranged in a regular structure in order that they provide an even level of background light. The luminaires within the array are often laid to a grid or lattice arrangement such that there is uniform spacing between them. The grid or lattice arrangement may, for example, be dictated by the structure of a false ceiling.
Each of the individual luminaires in such a lighting array is adapted such that it is able to communicate with the other luminaires over a wireless communication network, which is formed by an array of associated communication nodes. Each of the communication nodes in the network is located at the position of its associated luminaire in the lighting array. Hence, the spatial structures of the lighting array and communication network are equivalent.
The communication network provides a means by which the lighting array can be auto-commissioned post-installation. However, the individual nodes in the network are unable to provide their own position information, therefore, it is unknown which luminaire each communication node is associated with. Before the array can be commissioned, the spatial position of each node in the communication network must be established so that each node can be assigned to the correct luminaire.
The positions of the communication nodes are found by a trilateration process, which is based upon range data provided by the wireless communication network. The range data is provided in the form of range measurements taken between pairs of communication nodes in the wireless network. The calculation of a range between two nodes is derived directly from these range measurements, which are made using techniques like Received Signal Strength Indication (RSSI) or Time-of-Flight.
In the case of RSSI, the received strength of a radio signal exchanged between a pair of communication nodes is used to calculate the range between them. The strength of the transmitted signal decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. Hence, taking the wavelength into account, the distance between the pair of nodes may be calculated from the transmitted signal's attenuation at the receiving node.
In the case of Time-of-Flight measurements, the range between a pair of communication nodes is calculated by measuring the time taken for a radio signal to travel between them. It is known that radio signals travel at the speed of light, hence, an accurate measure of the time-of-flight between the pair of nodes enables an accurate calculation of the distance between them.
However, these types of range measurement are subject to error and, hence, the derived positions of the communication nodes often do not match exactly to positions on the grid or lattice arrangement on which the luminaires are arranged. There is, therefore, still some uncertainty as to which luminaire each node is associated with.
In order for the wireless lighting array to be successfully commissioned, the communication nodes must be assigned to their correct grid or lattice position, and hence luminaire, in the lighting array. If the communication nodes are assigned to a lattice position which does not correspond to their actual lattice position, the derived spatial structure of the communication network will be incorrect and, consequently, the lighting array will not function correctly.
In order to resolve such uncertainties in the positions of the communication nodes, the positions derived by trilateration may be compared with a template which defines the lattice positions of the luminaires in the lighting array. By this method, a communication node can be “snapped” to the closest luminaire to its derived position. Its new position can then be used as a reference point in the trilateration of further communication nodes. In this way, errors in the positions derived by the trilateration process are not accumulated.
However, there is a risk, with the above method, that individual communication nodes will be snapped to the wrong position, i.e. a position which does not correspond to their actual position. In this case, the use of that position to establish the positions of further communication nodes results in the accumulation of large errors. Such errors may become sufficiently large that they prevent the overall topology of the lighting array from being established.